Free-machining aluminum alloy

ABSTRACT

Various illustrative embodiments of a free-machining aluminum alloy composition and related methods are provided. The aluminum alloy composition can include an aluminum alloy and an effective amount of a graphitic material as a free-machining constituent. Free-machining means the graphitic material is capable of modifying the machining character of the aluminum alloy by affecting the generation of chip-shaped machining debris and/or the friction between the machining tool and the workpiece.

RELATED APPLICATIONS

This application claims the benefit, and priority benefit, of U.S. Provisional Patent Application Ser. No. 61/394,110, filed Oct. 18, 2010, titled “Free Machining Aluminum Alloy,” the disclosure of which is incorporated herein in its entirety.

BACKGROUND

1. Field of Invention

This invention relates generally to a free-machining aluminum alloy composition and related methods.

2. Description of the Related Art

Aluminum alloys are useful in a variety of applications. However, improving one property of an aluminum alloy without degrading another property often proves elusive. For example, it is difficult to increase the machinability of an alloy without also increasing the level of environmentally unfriendly heavy metals in the alloy. Other properties of interest for aluminum alloys include, for example, recyclability and chip formation during machining.

SUMMARY

Various illustrative embodiments of a free-machining aluminum alloy composition and related methods are provided herein. In certain illustrative embodiments, an aluminum alloy composition is provided. The composition can include an aluminum alloy and boron nitride as a free machining constituent. The aluminum alloy can be AA 6061. The amount of boron nitride in the composition can be greater than about 0.25% by weight. The composition can be essentially heavy metal free. The boron nitride can have a particle size in the range from about 5-150 microns. The composition can consist essentially of, in approximate weight %, from about 0.25-2.5% boron nitride; from about 0.5 to 2.0% magnesium; from about 0.5 to 2.0% silicon; from about zero to 2.0% copper; from about zero to 1.0% manganese; and the balance being aluminum, incidental elements and impurities. The boron nitride composition can be greater than about 3.0% by weight and can be used as a master alloy.

In another illustrative embodiment, a free machining, essentially heavy metal free aluminum alloy composition is provided. The composition can include an aluminum alloy and boron nitride in a weight % in the alloy composition such that the alloy composition exhibits reduced workpiece/tool friction during machining and promotes the generation of small chip-like machining debris.

In another illustrative embodiment, an aluminum alloy composition is provided. The composition can include an aluminum alloy and a graphitic material as a free machining constituent. The graphitic material can include boron nitride. The graphitic material can also include one or more materials from the group consisting from graphite, boron nitride, molybdenum disulfide and tungsten disulfide.

In another illustrative embodiment, a method of improving the free machining properties of an aluminum alloy is provided. A molten aluminum alloy can be supplied. The composition of the molten aluminum alloy can be adjusted by adding an effective amount of boron nitride so that the boron nitride occupies greater than about 0.25% by weight of the alloy. The alloy can be solidified so that the alloy is capable of being machined into a machined article.

In another illustrative embodiment, a method of making an aluminum alloy material is provided. A molten aluminum alloy can be supplied. Graphitic particles can be added to the molten aluminum alloy. The graphitic particles can be dispersed in the molten aluminum alloy. The molten aluminum alloy can be formed into a solidified aluminum alloy material. The molten aluminum alloy can be ultrasonically oscillated to disperse and/or wet the graphitic particles in the alloy. A gas can be injected into the molten metal to form microbubbles in the molten metal. The graphitic particles can be agglomerated on or around the microbubbles to enhance the dispersion of the graphitic particles in the molten metal. The graphitic particles can include boron nitride. The graphitic particles can also include one or more materials from the group consisting of boron nitride, graphite, tungsten disulfide and molybdenum disulfide.

In another illustrative embodiment, a method of making an aluminum alloy material is provided. A molten aluminum alloy can be supplied. Graphitic particles can be added to the molten aluminum alloy. The graphitic particles can be dispersed in the molten aluminum alloy, and a molten aluminum master alloy can be formed. The molten aluminum master alloy can be ultrasonically oscillated to disperse the graphitic particles in the master alloy. A gas can be injected into the molten metal to form microbubbles in the molten metal. The graphitic particles can be agglomerated around the microbubbles to enhance the dispersion of the graphitic particles in the molten master alloy. The graphitic particles can include boron nitride. The graphitic particles can also include one or more materials from the group consisting of boron nitride, graphite, tungsten disulfide and molybdenum disulfide. The graphitic particles can make up between about 3.0% and 50%, by weight, of the master alloy.

It is to be understood that the subject matter herein is not limited to the exact details of construction, operation, exact materials, or illustrative embodiments shown and described, as modifications and equivalents will be apparent to one skilled in the art. Accordingly, the subject matter is therefore to be limited only by the scope of the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing the dispersion of graphitic particles within an aluminum alloy in certain illustrative embodiments.

FIG. 2 is a micrograph showing a dispersion of boron nitride particles dispersed with aluminum alloy AA6061 in certain illustrative embodiments.

FIG. 3 is a front view of an ultrasonic oscillator for dispersing boron nitride particles in a molten aluminum alloy in certain illustrative embodiments.

FIG. 4 is a front view of a diffuser with gas injection for use with an ultrasonic oscillator to disperse boron nitride particles in a molten aluminum alloy in certain illustrative embodiments.

FIG. 5 is a photograph of conventional machined rod and machining chips of aluminum alloy AA6061-T6

FIG. 6 is a photograph of machined rod and machining chips of the aluminum alloy AA6061-T6 incorporating 1.0 wt % boron nitride.

DETAILED DESCRIPTION

Various illustrative embodiments of a free-machining aluminum alloy composition and related methods are provided herein. The aluminum alloy composition can include an aluminum alloy and an effective amount of a graphitic material as a free-machining constituent. Free-machining means the graphitic material is capable of modifying the machining character of the aluminum alloy by affecting the generation of chip-shaped machining debris and/or the friction between the machining tool and the workpiece. For example, in conventional free-machining alloys, a combination of low melting heavy metals forms a constituent phase upon solidification, creating globules of a low melting point compound in the alloy. A local increase in the alloy temperature (due to machining of an article made from the alloy, for example) brings the low melting point compound to a soft or liquid state. In this state, the low melting point compound loses its strength thereby facilitating the formation of small chips during machining. In certain illustrative embodiments, the presently disclosed subject matter avoids the need for such low melting point compounds and the contaminating effects of the use of such heavy metal additions in the aluminum alloy, while still facilitating the formation of small chips during machining that do not interfere with the machining process. In certain illustrative embodiments, the graphitic material can be one or more materials selected from the group consisting of carbon graphite, boron nitride, molybdenum disulfide and tungsten disulfide. All four of these materials are graphitic in structure, meaning they are composed of sheets of atoms held together by weak van der Waals forces. As a result, all four materials are relatively soft (Mohr Hardness values less than 3), are relatively strong lubricity agents, and can be used as dry lubricants at elevated temperatures.

In an illustrative embodiment, the graphitic material is boron nitride, as this compound is chemically inert to attack by molten aluminum and is a high temperature lubricant in forging and superplastic forming operations. Further, boron nitride is available in graphitic grades in the range of approximately 6 to 45 microns, a size range comparable to the lead and tin globules currently used in free-machining alloys. In an illustrative embodiment, boron nitride will be present in the aluminum alloy in flakes or particles in the range from approximately 5-150 microns in size, but the boron nitride can have any size, and have other properties, that would allow it to be dispersed throughout the aluminum alloy. In an illustrative embodiment (FIG. 1), an aluminum alloy 10 contains a dispersion of graphitic particles 20 to act as initiation sites for machining chip fracture and as in-situ lubrication for high speed machining operations. In a specific embodiment (FIG. 2), a photomicrograph of an aluminum alloy 40 (AA6061) is shown with a dispersion of boron nitride particles 30 within the matrix of alloy 40.

In an illustrative embodiment, the aluminum alloy composition is free or substantially free of heavy metal additions, where heavy metals refers to elements including, but not limited to, lead, bismuth, tin, or indium. The boron nitride (or other graphitic material) replaces all, or substantially all, of the heavy metals that would typically be added to the alloy to improve machining properties. In certain illustrative embodiments, the aluminum alloy is AA 6061; however, other machinable aluminum alloys such as, but not limited to, 6063, 2024, and 7075 can also be utilized without departing from the spirit and scope of the present subject matter. In an illustrative embodiment where the aluminum alloy is AA 6061, the graphitic material preferably comprises in the range from approximately 0.25%-3.0%, by weight, of the composition. Thus, the constituents of the free-machining AA 6061 aluminum alloy composition can include, for example, by approximate weight %: from 0.25%-3.0% boron nitride; from 0.5 to 2% magnesium; from 0.5 to 2.0% silicon; from zero to 2.0% copper; from zero to 1.0% manganese; and the balance being aluminum, incidental elements and impurities. Regardless of the aluminum alloy that is selected, the graphitic material should be present in the amount necessary to provide the desired level of free-machinability to the alloy composition.

In an illustrative embodiment, a master alloy containing the boron nitride (or other graphitic material) can be produced and this master alloy can be used in the subsequent casting of the free-machining aluminum alloy. A master alloy means an intermediate product comprising a concentrated form of the element or compound to be added to an alloy and diluted. In this illustrative embodiment, an aluminum master alloy is produced with a high loading of boron nitride (or other graphitic material) comprising in the range from approximately 3% to 50%, by weight, of the composition. This master alloy can then be added to a molten aluminum alloy in a subsequent operation to produce a free-machining alloy with a boron nitride (or other graphitic material) content comprising in the range from approximately 0.25%-3.0%, by weight, of the composition.

Methods of making a free-machining aluminum alloy composition or a master alloy for making a free-machining aluminum alloy composition are also described herein. In certain illustrative embodiments, a molten metal can be provided, wherein the molten metal can include liquefied aluminum as well as other constituents. Boron nitride particles (or other graphitic material) can be dispersed within the molten metal. A dispersing aid, such as an ultrasonic oscillator, can be used to disperse the boron nitride particles throughout the molten metal. (see FIG. 3). In a specific embodiment, the ultrasonic oscillator 50 can be utilized and combined with mild vortexing agitation of the molten metal, or otherwise assist in dispersing the boron nitride particles 20. Further, an inert gas, such as argon gas 80, can be injected into the molten metal to form microbubbles 70 within the metal. (See FIG. 4). A diffuser 60 can be used to spread out or scatter the microbubbles 70 within the molten metal and ensure that the bubble sizes remain sufficiently refined to retard rapid rising through buoyancy. The boron nitride particles 20 will preferably agglomerate on or around the microbubbles 70 to enhance the distribution and dispersion of the boron nitride particles 20 in the molten metal. A stirrer or other similar agitating device can also be utilized to further disperse the boron nitride particles 20 in the molten metal. After the boron nitride particles have been dispersed, a solidified aluminum alloy material with a dispersion of particles can be formed from the molten metal.

Methods of improving the free machining properties of an aluminum alloy are also provided. In an illustrative embodiment, a molten aluminum alloy can be provided. The composition of the molten aluminum alloy can be adjusted by adding an effective amount of boron nitride (or other graphitic material) so that the material occupies greater than about 0.25% by weight of the alloy. The graphitic material can be dispersed throughout the molten aluminum alloy. The alloy can then be solidified so that it is capable of being machined into a machined article.

The various illustrative embodiments of the free-machining aluminum alloy described herein have a number of desirable properties. For example, the alloy has improved machinability as compared to conventional alloys. The present alloy has relatively high lubricity at the tool/workpiece interface, which leads to reduced tool wear. Further, the alloy can be classified as Class A in machining behavior, which means that it demonstrates relatively low workpiece/tool friction and allows for small chip formation, thus avoiding a build-up of machine turnings in automated machining operations. Also, the free-machining alloy does not contain any, or virtually any, heavy metals such as lead, tin or bismuth, and so it can be recycled in the general scrap stream or disposed of as environmentally friendly, non-hazardous waste. This fully recyclable, heavy metal free, easy to machine aluminum alloy can be formed into a billet product or cast plate product for jig and mold plates, and can ultimately be used in applications such as valve blocks, connectors and fasteners, master cylinders and brake pistons, antilock braking system (ABS) components and sporting goods.

Example

Fabrication: A suspension of aluminum of the AA6061 composition was melted and blended with 1.0 wt %-325 mesh sized boron nitride powder (D10=4.8 microns; D50=9.6 microns; D90=17.6 microns). The molten suspension was subjected to ultrasonic oscillation at 1.5 to 2.5 kW power for 3 minutes to disperse and wet the boron nitride powder and subsequently cast. Extrusion billets of 30 mm diameter were machined from the cast product and homogenized at 570° C. for 3 hours and allowed to air cool. Billet specimens were heated to 476° C. and extruded to a rod diameter of 14 mm. Following solutionizing at 538° C. for 5 minutes, rod specimens were direct quenched into ambient temperature water. Lastly, the solutionized specimens were subjected to the standard 6061 aging practice of 170° C. for 8 hours to bring the specimens to a T6 temper.

Machining Trials: Standard machinability tests were conducted on baseline specimens of AA6061-T6 and on the AA6061-T6 specimens produced using the 1.0 wt % boron nitride addition. 14 mm diameter rods were turned on an automated lathe at 1500 rpm with a feed rate of 0.076 mm/rev. Cut depths were varied between 0.25 mm and 1.8 mm and chips were collected from all conditions. Chips per gram, a standard metric for machinability, were made for both specimens at different machining conditions, with high chip counts generally being associated with superior machinability. The baseline AA6061-T6 specimens produced less than 10 chips per gram, typical of Class C alloys (FIG. 5). The AA6061-T6 specimens incorporating the 1.0% boron nitride addition produced over 250 chips per gram (FIG. 6), chip counts associated with Class B heavy metal containing free-machining alloys such as 6060-T6 and Class A heavy metal containing free machining alloys such as 2011-T6.

It is to be understood that the subject matter herein is not limited to the exact details of construction, operation, exact materials, or illustrative embodiments shown and described, as modifications and equivalents will be apparent to one skilled in the art. Accordingly, the subject matter is therefore to be limited only by the scope of the appended claims. 

1. An aluminum alloy composition comprising: an aluminum alloy; and boron nitride as a free machining constituent.
 2. The aluminum alloy composition of claim 1, wherein the aluminum alloy is AA
 6061. 3. The aluminum alloy composition of claim 1, wherein the amount of boron nitride in the composition is greater than 0.25% by weight.
 4. The aluminum alloy composition of claim 1, wherein the composition is essentially heavy metal free.
 5. The aluminum alloy composition of claim 1, wherein the boron nitride has a particle size in the range from 5-150 microns.
 6. The aluminum alloy composition of claim 1, consisting essentially of, in approximate weight %: from 0.25-2.5% boron nitride; from 0.5 to 2.0% magnesium; from 0.5 to 2.0% silicon; from zero to 2.0% copper; from zero to 1.0% manganese; and the balance being aluminum, incidental elements and impurities.
 7. The aluminum alloy composition of claim 1 wherein the boron nitride composition is greater than 3.0% by weight and is used as a master alloy.
 8. A free machining, essentially heavy metal free aluminum alloy composition comprising: an aluminum alloy; and boron nitride in a weight % in the alloy composition such that the alloy composition exhibits reduced workpiece/tool friction during machining and promotes the generation of small chip-like machining debris.
 9. An aluminum alloy composition comprising: an aluminum alloy; and a graphitic material as a free machining constituent.
 10. The aluminum alloy composition of claim 9, wherein the graphitic material comprises boron nitride.
 11. The aluminum alloy composition of claim 9, wherein the graphitic material comprises one or more materials from the group consisting from graphite, boron nitride, molybdenum disulfide and tungsten disulfide.
 12. A method of improving the free machining properties of an aluminum alloy comprising: providing a molten aluminum alloy; adjusting the composition of the molten aluminum alloy by adding an effective amount of boron nitride so that the boron nitride occupies greater than 0.25% by weight of the alloy; and solidifying the alloy so that the alloy is capable of being machined into a machined article.
 13. A method of making an aluminum alloy material, the method comprising: providing a molten aluminum alloy; adding graphitic particles to the molten aluminum alloy; dispersing the graphitic particles in the molten aluminum alloy; and forming the molten aluminum alloy into a solidified aluminum alloy material.
 14. The method of claim 13, further comprising ultrasonically oscillating the molten aluminum alloy to disperse and wet the graphitic particles in the alloy.
 15. The method of claim 13, further comprising injecting a gas into the molten metal to form microbubbles in the molten metal; and agglomerating the graphitic particles on or around the microbubbles to enhance the dispersion of the graphitic particles in the molten metal.
 16. The method of claim 13, wherein the graphitic particles comprise boron nitride.
 17. The method of claim 13, wherein the graphitic particles comprise one or more materials from the group consisting of boron nitride, graphite, tungsten disulfide and molybdenum disulfide.
 18. A method of making an aluminum alloy material, the method comprising: providing a molten aluminum alloy; adding graphitic particles to the molten aluminum alloy; dispersing the graphitic particles in the molten aluminum alloy; and forming a molten aluminum master alloy.
 19. The method of claim 18, further comprising ultrasonically oscillating the molten aluminum alloy to disperse the graphitic particles in the master alloy.
 20. The method of claim 18, further comprising injecting a gas into the molten metal to form microbubbles in the molten metal; and agglomerating the graphitic particles around the microbubbles to enhance the dispersion of the graphitic particles in the molten master alloy.
 21. The method of claim 18, wherein the graphitic particles comprise boron nitride.
 22. The method of claim 18, wherein the graphitic particles comprise one or more materials from the group consisting of boron nitride, graphite, tungsten disulfide and molybdenum disulfide.
 23. The method of claim 18, wherein the graphitic particles comprise between 3.0% and 50%, by weight, of the master alloy. 